The invention concerns a radio-controlled wristwatch and in particular to an antenna therefor.
In a timepiece of the general kind set forth, as disclosed in German Document 93 18 224.4, a longwave antenna consisting of a coil and a cylindrical ferrite core for the reception of the encoded absolute time information is arranged beside a printed circuit board of an electronic module in the wristwatch casing. The antenna is parallel to a strap connection in a hollow space provided in a thickened horn base portion. Particularly in the case of small wristwatch casings such as for ladies' wristwatches, that necessitates a very short ferrite bar and a correspondingly low level of antenna sensitivity. It is for that reason that the reception of usable time telegrams is possible only under very good receiving conditions or with an extremely sensitive receiver. However, it is precisely when the receiver has a high level of sensitivity that such time telegrams can easily be adversely affected in terms of their decodability by external interference sources or by interference sources which are internal to the timepiece, such as in particular the high frequency clock generator for operating the processor.
A greater degree of immunity from interference as a result of a higher level of antenna sensitivity and a reduction in the required degree of receiver sensitivity is achieved by means of a larger core mass for the magnetic longwave antenna. Ganter et al. U.S. Pat. No. 5,430,693 (corresponding to EP 0 649 076-A3) provides a core which is no longer a cylindrical coil carrier but rather a ferrite plate which substantially fills the casing diameter. The plate serves at the same time as a carrier for mechanical and electrical components and for the circuitry of the electronic timepiece module. In that arrangement the antenna coil is arranged on a lateral projection of the ferrite plate. A shaped ferrite component of that kind however is expensive to produce and requires a high degree of dimensional precision for dimensionally accurately receiving the other mechanical and electrical components of the timepiece. Also, a plate, especially a plate with locally weakened portions for receiving the timepiece components and the antenna coil, is in great danger of fracturing in a situation involving a shock loading.
For installation of an antenna in a space of angled configuration in small timepiece casings, Ganter et al. U.S. Pat. No. 4,947,179 (corresponding to EP 0348 636 A1) provides that the antenna be formed of for example flexible strips which are displaceable relative to each other and which comprise soft-magnetic materials of high permeability such as in particular amorphous metals in sheet form. The strips are designed in an angled configuration substantially in the shape of an L, U, or Z, depending on the local circumstances of installation, in order to make optimum use of the spaces available in the timepiece casing for installing the antenna and in order to embody different reception orientations. A flexible core stack comprising individual, magnetically effective strips can thereafter also be curved or bent to form a portion of a ring in order to represent the longwave antenna, as part of a bangle. In contrast, in the case of a radio-controlled wristwatch, provision is made for displacing the antenna into its watch bracelet. Disadvantages involving the provision of angled spaces in the casing are the complicated shaping required and the resulting high level of assembly expenditure, as well as the comparatively short effective length (as measured by way of projection) of such a core geometry which extends in an angled configuration. While the installation of a flexible stack of strips in the wristwatch bracelet admittedly produces optimum reception conditions in regard to the distance from interference sources which are internal to the wristwatch and in regard to the effective core length, the flexible connection from the antenna coil to the receiving circuit by way of the bracelet connection to the wristwatch casing is in practice highly susceptible to wear.
In consideration of those facts an object of the present invention is to provide a magnetic longwave antenna for a radio-controlled wristwatch which represents an optimum compromise between the previously known extremes in regard to sensitivity to interference and antenna sensitivity and which is easy to handle in production and which is distinguished by a high level of mechanical stability.